Clutch protection system

ABSTRACT

A clutch protection system to guard against overheating due to excessive slippage in a friction clutch (12). The system which is particularly suitable for use with a tractor power take-off clutch includes sensors (18, 19) to provide signals indicative of the input and output speeds of the clutch and an indicating device (56) which receives a first signal (I) indicative of the clutch input speed and a second signal (X) indicative of either the clutch output speed when the clutch engagement pressure is applied or the clutch input speed when the clutch engagement pressure is not applied. The indicating device has a signal level indicative of the difference between the first and second signals minus a signal indicative of the cooling effect on the clutch due to its own rotation. A comparator (56) compares the signal level of the indicating device with a predetermined signal level indicative of, for example, a potentially damaging clutch operating condition and actuates a warning device (40) and disengages the clutch when the predetermined clutch operating condition occurs.

TECHNICAL FIELD

This invention relates to friction clutches and in particular toprotection systems which guard against overheating of such clutches dueto excessive slippage.

There are many applications where the protection of clutches againstdamage due to excessive slippage are desirable. One such application isa clutch in a tractor power take-off (PTO) drive line used to connectand disconnect a power take-off (PTO) shaft from the tractor engine.

It is an object of the present invention to provide a clutch protectionsystem to guard against overheating due to excessive slippage which issuitable for the protection of a PTO drive line clutch.

DISCLOSURE OF THE INVENTION

According to the present invention there is provided a clutch protectionsystem to guard against overheating due to excessive slippage in afriction clutch having an engagement means which applies an engagementpressure to the clutch, the system including sensing means to providesignals indicative of the input and output speeds of the clutch; anindicating device which receives a first signal indicative of the clutchinput speed and a second signal indicative of either the clutch outputspeed when the clutch engagement pressure is applied or the clutch inputspeed when the clutch engagement pressure is not applied, the indicatingdevice having a signal level indicative of the difference between thefirst and second signals minus a signal indicative of the cooling effecton the clutch due to its own rotation; comparison means for comparingthe signal level of the indicating device with a predetermined signallevel indicative of a clutch operating condition; and response meansresponsive to a predetermined comparison result to action a warningdevice or disingage the clutch.

The operation of the indicating device can be expressed mathematicallyby stating, for example, that the signal level of the indicating deviceis proportional to I-S.X (where I is the input speed signal, S isscaling factor which takes account of the cooling effect on the clutchdue to its own rotation, and X is the output speed signal when theclutch engagement pressure is applied and the input speed signal whenthe clutch engagement pressure is not applied) and this signal level iscompared in the comparison means with the predetermined signal level.

For example, the comparison means may compare the signal level of theindicating device with first and second predetermined signal levelsindicative of different levels of clutch slippage and the response meanswill action a warning if the signal level of the indicating deviceexceeds the first predetermined signal level (indicating, for example,that a higher than desirable but still tolerable slippage level exists)and will action a further warning and disengage the clutch if the signallevel of the indicating device exceeds the second predetermined signallevel (indicating, for example, that slippage has risen to a level abovewhich it cannot be allowed to rise without a serious risk of permanentdamage to the clutch).

Determination of the appropriate predetermined signal levels for a givenclutch design and/or installation is easily achieved by a systemdesigner who will know (or can determine by testing) the maximum levelof slip which the clutch can be subjected to without incurring permanentdamage (referred to above as the second predetermined level). Thedesigner can then set his own early warning of potentially dangerousslip levels (the first predetermined level referred to above). Thisearly warning level of slippage could be say half the maximum level ofslippage or some other proportion depending both on the inclination ofthe designer to play safe and also on the clutch design and itsinstallation. For example, as discussed below, the Applicant has foundthat with a particular tractor PTO drive line clutch installation thesecond predetermined level could be appropriately set at the signallevel reached by the indicating device in two seconds with the clutchfully engaged and with the clutch input running at 2200 revs/min. andthe clutch output held stationary. The appropriate corresponding firstpredetermined level was found to be half the second predetermined level.

As explained previously once the predetermined signal level or levelshave been set by the system designer the scaling factor S required tosimultate the cooling effect of the clutch for a given clutch and/orinstallation can also be determined. This is achieved, for example, bythe designer deciding what he considers to be an acceptable operatingtemperature for the clutch and then determining how long it will takethe clutch temperature to fall to this chosen temperature from thetemperature achieved at the second predetermined signal level with theclutch rotating at a given speed, say its rated speed, and with noslippage in the clutch. This ability of the clutch to cool at given ratedue to its own rotation is then simulated by scaling factor S in termsof a rate of reduction in the signal level of the indicating devicewhich will reduce the signal level of the indicating device from itssecond predetermined signal level to zero in the time taken for theclutch to reach the chosen acceptable operating temperature from thetemperature achieved at the second predetermined signal level. Forexample, in the tractor PTO drive line clutch installation referred toabove the scaling factor, as explained below, was chosen so as to reducethe indicating device signal level to zero in three minutes with theclutch input running at 1800 revs/min and no clutch slippage.

The clutch protection system of the present invention is particularlysuitable for use in conjunction with a clutch engagement control systemas described and claimed in the Applicants co-pending published UKpatent application No. 2156938A in which clutch engagement is effectedby fluid pressure supplied from a pressure source via a solenoidoperated valve controlled in accordance with target clutch outputacceleration criteria using the so-called pulse width modulationtechnique. By oscillating the control valve between its "off" and "on"conditions at a high frequency the rate of build-up of clutch-engagingpressure can be controlled so that the acceleration of the clutch outputmember follows the desired pattern as described and claimed in the abovereferred to co-pending application.

When the clutch protection system of the present invention is used inconjuntion with a clutch engagement control system in accordance withco-pending published UK patent application No. 2156938A X is taken asthe output speed signal during the "on" condition of the solenoidoperated valve and the input speed signal during the "off" condition ofthe valve.

Conveniently the indicating device may comprise a digital counter whichreceives a first series of count pulses indicative of clutch input speedand a second series of count pulses which is either indicative of clutchoutput or input speed depending on whether the clutch engagementpressure is applied or not, the arrangement being such that the countlevel in the counter represents the difference between the first andsecond series of count pulses minus a scaled proportion of said secondseries of count pulses to simulate the cooling effect

DESCRIPTION OF THE DRAWINGS

One embodiment of the present invention as applied to a tractor PTOdrive line clutch will now be described, by way of example only, withreference to the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a clutch protection system inaccordance with the present invention, and

FIG. 2 is a logic diagram for a clutch protection system in accordancewith the present invention

BEST MODE OF CARRYING OUT THE INVENTION

Referring to FIG. 1 this shows a diagrammatic representation of a PTOdrive line for a tractor in which an engine 10 drives an implement 11via a PTO clutch 12 and a two-speed gear set 24. Clutch 12 is engaged byfluid pressure supplied by a pump 13 via a solenoid-operated valve 14and is disengaged by spring means (not shown) when the clutch is ventedto a sump 15 by valve 14.

The precise constructional details of clutch 12 form no part of thepresent invention and will not therefore be described in detail. Anyknown form of PTO clutch of the necessary torque capacity could be used.For example, the multiplate type of PTO clutch currently used on theMassey-Ferguson 2000 Series tractors in which pressurisation of anannular chamber 12a engages the clutch.

The solenoid valve 14 is also of known form and has two positions. Whenin the "off" position shown in which its solenoid 14a is not energisedthe clutch is disengaged since chamber 12a is vented to sump 15. When inthe "on" position the solenoid 14a is energised and the valve isdisplaced to its other position in which the clutch is connected withthe pump 13 to pressurise chamber 12a and engage the clutch.

Two sensing devices 18 and 19 are provided to sense the speed ofrotation of the shaft 16 which acts as the input into clutch 12 andshaft 17 which acts as the output of the clutch. These sensing devicesmay be of any suitable form and again the precise constructional detailsof these devices do not form part of the present invention. For example,the sensing devices may be of the known electromagnetic type in which aspecially provided tooth disc rotates with each shaft and the passage ofthe teeth of this disc past a coil induces a pulsating voltage in thecoil circuit whose frequency of pulsation is a measure of the speed ofrotation of the associated shaft.

Ideally, the provision of special toothed discs to provide the clutchinput and output speed indications is to be avoided and as shown, forexample, the starter motor teeth 21 on the outer periphery of the engineflywheel 20 can be utilised as part of the sensing device 18 and theteeth 22 on a gear wheel 23 which forms part of two speed gear set 24can be utilised as part of sensing device 19.

The pulse counts coming from sensing devices 18 and 19 are fed as inputsinto a micro-processor control circuit indicated diagrammatically by box25 in FIG. 1.

Although control circuit 25 operates digitally, in order to facilitatethe understanding of the operation of the circuit, its main processingoperations are diagrammatically illustrated within box 25 in FIG. 1using a block diagram electrical circuit anology.

Control circuit 25 receives an input signal from a driver-operatedon-off switch indicated diagrammatically at 26 which the driver closeswhen he wishes to engage the PTO clutch 12. A warning light 40 isilluminated in either a continuous display or flashing mode by thecontrol circuit 25 via a driving unit 50 and an output line 52 and thesolenoid 14a of valve 14 communicates with circuit 25 via driving unit51 and output line 53.

The control circuit 25 includes a clutch engagement control system inaccordance with the Applicant's previously referred to UK patentapplication No. 2156938A in which during the clutch engagement processthe solenoid 14a of control valve 14 is oscillated at a high frequencyusing the so-called pulse width modulation technique in order that therate of build-up of clutch engaging pressure is controlled by valve 14so that the acceleration of the clutch output shaft 17 flows apredetermined pattern to ensure a progressive and shock-free engagementof the clutch 12.

As described and claimed in the above referred to patent application,the pulse width modulation function (designated PWM in FIG. 1) receivesthe pulse counts coming from sensors 18 and 19 via lines 54 and 55 andscaling devices M and N which take account of any difference in thenumber of teeth on flywheel 20 and on gear wheel 23. The PWM functionsets target accelerations for the clutch output shaft 17 dependent onthe ratio of the clutch input and output speeds in accordance with apredetermined acceleration to speed-ratio relationship. The PWM functioncompares the target acceleration levels of the clutch output shaft withthe actual acceleration levels achieved by the output shaft and adjuststhe rate of increase of the clutch engagement pressure by oscillatingthe solenoid 14a of control value 14 via line 53 so that the targetlevels are achieved. If further details of the operation of the PWMfunction are desired, the reader is referred to UK patent applicationNo. 2156938A.

Control circuit 25 also includes a clutch protection system inaccordance with the present invention having an indicating device in theform of counter 56 which indicates the heating effect experienced byclutch 12 due to slippage. The counter 56 has a positive input whichreceives the scaled signals from line 54 and a negative input whichreceives either the scaled input from line 55 or line 54 depending onwhether solenoid 14a is energised or not. This switching of the negativecounter input between lines 54 and 55 is indicated diagrammatically inFIG. 1 by a switching means Q which is actuated dependent on theoperation of solenoid 14a as indicated by dotted line 57. The negativecounter input whether from line 54 or 55 is also scaled in anappropriate scaling device by a scaling factor S (referred to previouslyand also discussed below) which takes account of the cooling effect onthe clutch due to its own rotation.

It will be appreciated from the above that the signal level in counter56 is equal to I-S.X where I is the clutch input speed signal count fromline 54, S is the scaling factor discussed above, and X is either theoutput speed signal count from line 55 or the input speed signal countfrom line 54 depending on whether solenoid 14a is actuated or not. Amathematical justification for the use of a counter signal level of thisform to represent a practical approximation to the heating effectexperienced by the clutch 12 due to slippage is given later below.

Counter 56 also performs a comparison function by comparing its currentsignal level with first and second predetermined count levels indicativeof different levels of clutch slippage. The first signal levelcorresponds to a level of slippage which is higher than desirable butstill tolerable whilst the second signal level corresponds to a level ofslippage above which slippage cannot be allowed to rise without aserious risk of permanent damage to the clutch. Counter 56 produces anoutput signal (represented at line 58 in FIG. 1) when the counter signallevel is above the first level and below the second level and a furtheroutput signal (represented by line 59 in FIG. 1) when the counter signallevel reaches the second level.

As previously stated the Applicants have found in a particular tractorPTO driveline which they have tested that a second signal levelequivalent to the signal generated in the counter in two seconds withthe clutch fully engaged and with the clutch input running at 2200revs/min and the clutch output held stationary was appropriate. It wasalso found appropriate to make the first signal level equal to half thesecond level.

When an output appears at line 58 the light 40 which was previouslycontinuously lit following the closing of switch 26, is flashed at a lowfrequency using a frequency controller FR1 to warn the tractor operatorthat clutch slippage levels are rising to higher than desirable levels.If an output appears at line 59 solenoid 14a of valve 14 is deactivatedby a switch 60 to disengage the clutch 12 and prevent permanent damageto the clutch and a second frequency controller FR2 flashes light 40 ata higher frequency to warn the operator that the protection system hasdisengaged the PTO clutch.

For simplicity of illustration light 40 is shown in FIG. 1 as beingcontrolled via a control device L from which emerges either a signal forconstant illumination of the light when switch 26 is closed or a signalfrom frequency controller FR1 or FR2 to flash the light when the countersignal level reaches the first and second signal levels as describedabove.

The counter 56 operates on an 80 ms (milliseconds) cycle time the sameas the PWM function. Thus the counter signal level is updated every 80ms so that the current counter signal level at any moment is equal tothe value of I-S.X for the previous 80 ms time period. The countersignal level is then compared with the first and second predeterminedsignal levels at regular intervals, for example, every 0.5 seconds.

As will be appreciated when the clutch 12 has been disengaged by switch60 re-engagement of the clutch will commence under the control of thePWM function once switch 60 has closed again. Switch 60 will close assoon as the current counter signal has fallen below the secondpredetermined signal level. Similarly flashing of light 40 will stop assoon as the current counter level falls below the first predeterminedsignal level.

The above described operation of the protection system isdiagrammatically illustrated in FIG. 2 which is a basic logic diagramfor the operation of the system.

As indicated in box 70 the input speed pulses from sensor 18 are addedinto counter 56. Dependent on whether the clutch 12 is engaged, which isdetermined by decision box 71 which ascertains whether solenoid 14a isenergised or not, either the output pulses from sensor 19 (see box 72)or the input pulses from sensor 18 (see box 73) are multiplied by thescaling factor S and subtracted from the count level in counter 56 (seebox 74). The level in counter 56 is compared with the first and secondpredetermined signal levels (see decision boxes 75 and 76 respectively).If the current count level is below the first predetermined level, asindicated by logic line 77, no action is taken and the entire logicoperation is then repeated. If the current count level is above thefirst predetermined level but below the second level, as indicated bylogic line 78, the warning light 40 is flashed, as indicated by outputbox 79. If the current count level is above the second predeterminedlevel, as indicated by logic line 80 the clutch 12 disengaged and light40 is flashed at a higher frequency, as indicated by output box 81.

A mathematical analysis of the factors affecting the heating effectexperienced by clutch 12 during use of the PTO drive line reveals thefollowing:

    The power dissipated (PD) in the clutch=Torque (wI-wO)

Where wI and wO are the input and output angular velocities of theclutch.

The cooling rate of the clutch due to its own rotation is a function ofat least wI, wO and the temperature of the clutch.

If we assume as an approximation that the input angular velocity andtemperature are substantially constant then the cooling rate isapproximately equal to K.wO where K is a constant.

Thus the heating effect over the time period t can be written as:##EQU1## If we assume a substantially constant torque K' then: ##EQU2##It wi11 be observed that ##EQU3## are in effect the clutch input anoutput shaft speed counts which we will designated as I and Xrespectively.

Therefore the heating effect is proportional to

    1-(I+K/K') X.

That is heating effect is proportional to I-S.X where S is the scalingfactor (1+K/K').

Thus the signal level I-S.X generated in counter 56 provides a signalwhich is sufficiently accurately proportional to the heating effectgenerated in the PTO clutch 12 to enable a practical clutch protectionsystem to be designed around this signal.

As explained previously once the predetermined signal level or levelshave been set by the system designer the scaling factor S required tosimultate the cooling effect of the clutch for a given clutch and/orinstallation can also be determined. This is achieved, for example, bythe designer deciding what he considers to be an acceptable operatingtemperature for the clutch and then determining how long it will takethe clutch temperature to fall to this chosen temperature from thetemperature achieved at the second predetermined signal level with theclutch rotating at a given speed, say its rated speed, and with noslippage in the clutch. This ability of the clutch to cool at given ratedue to its own rotation is then simulated by scaling factor S in termsof a rate of reduction in the signal level of the indicating devicewhich will reduce the signal level of the indicating device from itssecond predetermined signal level to zero in the time taken for theclutch to reach the chosen acceptable operating temperature from thetemperature achieved at the second predetermined signal level. Forexample, in the tractor PTO drive line clutch installation referred toabove the scaling factor was chosen so as to reduce the indicatingdevice signal level to zero in three minutes with the clutch inputrunning at 1800 revs/min and no clutch slippage.

If in a given drive line installation 60 pulses/rev emerge from sensingdevice 18 and 30 pulses/rev emerge from sensing device 19 scaling deviceM will need to scale its count rate by a factor of 2 to take account ofthe situation.

Given that the second signal level is equal to the signal generated inthe counter in two seconds when the clutch input is running at 2200rev/min and the clutch output is stationary this will give a reading of##EQU4## pulses for the second signal level.

If, for example, the counter 56 is an eight bit binary register it willhave a maximum count level of 256. Thus the pulse count rate coming fromsensing devices 18 and 19 needs to be scaled accordingly. In theparticular example chosen scaling devices M and N can therefore also bearranged to scale the pulse rates from sensing devices 18 and 19 by afactor of say 16 (2⁴) which is easy to achieve and which will give asecond signal level of 138 (that is ##EQU5## in the counter.

Given that the scaling factor S is chosen in the example quoted above,so as to reduce the counter level from its second signal level of say138 as discussed above to zero in 3 mins. with the clutch input runningat 1800 revs/min it will be observed this 3 min period will result in3×1800×30 pulses from the input sensing means 18.

Thus the number of pulses per count is ##EQU6## Thus if 1 count issubtracted from the counter reading every 1174 pulses from the sensingmeans 18 the required scaling factor S is achieved.

Although the invention has been described above in terms of a digitalcounter 56 forming part of a micro-processor control circuit 25 it willbe appreciated that the invention can be realised using an analogueapproach in which, for example, the signals from sensing devices 18 and19 are continuously read as voltages and the resultant voltage inaccordance with the algebraic relationship I-S.X gives a continuousindication of the heating effect being experienced by the clutch 12.

In an alternative configuration the early warning of tolerable buthigher than desirable levels of slippage (provided by the firstpredetermined signal level) is not employed and the system is simplyarranged to disengage the clutch when the previously described secondpredetermined signal level is reached. When the system is configured inthis manner the warning light 40 is continuously lit when the operatorcloses switch 26 and commences to flash when the clutch is disengaged bythe system to warn the operator that the system has disengaged theclutch.

The present invention thus provides a simple but effective protectionsystem for a clutch to guard against overheating due to excessiveslippage which is particularly suitable for the protection of a PTOdrive-line clutch.

I claim:
 1. A clutch protection system to guard against overheating dueto excessive slippage in a friction clutch (12) having an engagementmeans which applies an engagement pressure to the clutch, the systembeing characterised by the inclusion of sensing means (18,19) to providesignals indicative of the input and output speeds of the clutch; anindicating device (56) which receives a first signal (I) indicative ofthe clutch input speed and a second signal (X) indicative of either theclutch output speed when the clutch engagement pressure is applied orthe clutch input speed when the clutch engagement pressure is notapplied, the indicating device having a signal level indicative of thedifference between the first and second signals minus a signalindicative of the cooling effect on the clutch due to its own rotation;comparison means (56) for comparing the signal level of the indicatingdevice with a predetermined signal level indicative of a clutchoperating condition; and response means responsive to a predeterminedcomparison result to actuate a warning device (40).
 2. A clutchprotection system according to claim 1 for use with a friction clutch(12) in which the engagement means comprises a fluid pressure operatedclutch actuator (12a) fed with pressurised fluid via a solenoid operatedvalve (14) which is oscillated at high frequency between "off" and "on"conditions during clutch engagement to control the rate of build up ofpressure in the actuator using a pulse width modulation technique, thesystem being characterised in that the second signal (X) received by theindicating device (56) is taken as the clutch output speed signal duringthe "on" condition of the solenoid operated valve and as the clutchinput speed signal during the "off" condition of said valve.
 3. A clutchprotection system according to claim 1 characterised in that thecomparison means (56) compares the signal level of the indicating device(56) with first and second predetermined signal levels indicative ofdifferent levels of clutch slippage and the response means actions awarning (40) if the signal level of the indicating device exceeds thefirst predetermined signal level and actions a further warning anddisengages (60) the clutch (12) if the signal level of the indicatingdevice (56) exceeds the second predetermined signal level.
 4. A clutchprotection system according to claim 1 characterised in that theindicating device signal level used to action disengagement of theclutch is set at the signal level reached by the indicating device (56)in two seconds with the clutch (12) fully engaged and with the clutchinput (16) running at 2200 revs/min and the clutch output (17) heldstationary.
 5. A clutch protection system according to claim 1characterised in that the signal (S) indicative of the cooling effect ofthe clutch (12) due to its own rotation is set at a level such that itwill reduce the counter signal level from the clutch disengagement levelto zero in three minutes with a clutch input speed of 1800 rev/min andno clutch slippage.
 6. A tractor power take-off drive including afriction clutch (12) having a protection system according to claim
 1. 7.A clutch protection system according to claim 1 characterised in thatthe response means disengages the clutch in response to a predeterminedcomparison result.
 8. A clutch protection system according to claim 1characterised in that the indicating device comprises a digital counter(56) which receives a first series of count pulses (I) indicative ofclutch input speed and a second series of count pulses (X) which isindicative of either the clutch output speed when the clutch engagementpressure is applied or the clutch input speed when the clutch engagementpressure is not applied, the arrangement being such that the count levelin the counter represents the first series of count pulses minus theproduct of the second series of count pulses and a scaling factor (S)which simulates said cooling effect.
 9. A clutch protection system toguard against overheating due to excessive slippage in a friction clutch(12) having an engagement means which applies an engagement pressure tothe clutch, the system being characterised by the inclusion of sensingmeans (18,19) to provide signals indicative of the input and outputspeeds of the clutch; an indicating device (56) which receives a firstsignal (I) indicative of the clutch input speed and a second signal (X)indicative of either the clutch output speed when the clutch engagementpressure is applied or the clutch input speed when the clutch engagementpressure is not applied, the indicating device having a signal levelwhich represents the first signal minus the product of the second signaland a scaling factor (S) which simulates the cooling effect on theclutch due to its own rotation; comparison means (56) for comparing thesignal level of the indicating device with a predetermined signal levelindicative of a clutch operating condition; and response meansresponsive to a predetermined comparison result to actuate a warningdevice (40).